This invention relates to hydrotalcite compound which has excellent infrared absorption ability and a characteristic property of exhibiting excellent light-transmission when it is contained in agricultural film; a process for producing the same; infrared absorbing agent containing said hydrotalcite compound as the active ingredient; and agricultural film which contains said infrared absorbing agent.
Agricultural films have been widely used for greenhouse cultivation or tunnel cultivation of agricultural products. Those agricultural films are required to concurrently exhibit good light transmission and heat insulating property. That is, temperatures within a greenhouse or tunnel which are raised by the daytime sunbeams of 0.29-4.3 xcexcm in wavelength rapidly drop in night, in particular, clear weather night, due to radiational cooling. Such rapid temperature drop inside a greenhouse or tunnel incurs adverse effect on growth of crops. While various causes are considered to induce the rapid temperature drop, there is an opinion that heat radiation from surface of the earth or cultivated plants to the outside atmosphere (radiation as long wavelength infrared rays) in nights is the reason for the temperature drop. According to that view, the heat radiation is calculated, using Planck""s formula, i.e., the following formula (3), as black body radiation energy:
Excexxc2x7dxcex(erg. secxe2x88x921.cmxe2x88x922) Excexxc2x7dxcex=2xcfx80hC{circumflex over ( )}2/[xcex{circumflex over ( )}5{e{circumflex over ( )}(hC/xcexkT)xe2x88x921}]xc2x7dxcexxe2x80x83xe2x80x83(3)
in which
xcex: wavelength
h: Planck""s constant
C: velocity of light in vacuum
k: Boltzmann""s constant
T: absolute temperature.
From the calculation, it is explained that the rays of wavelengths within the infrared region, in particular, infrared rays (black body radiation energy) of 400-2000 cmxe2x88x921, the maximum being at 1000 cmxe2x88x921, are said to be emitted within the temperature range of from 30 to xe2x88x9210xc2x0 C. to induce the temperature drop.
For preventing such rapid temperature drop inside a greenhouse or tunnel, heat insulating film having infrared-absorbing ability is used. Such heat insulating film is provided by either using, as thermoplastic resin which is the base material, the one having infrared absorbing ability itself or the one to which a substance having ability to absorb infrared rays (in particular, rays of wave-lengths ranging 400-2000 cmxe2x88x921), i.e., an infrared absorbing agent, is blended so as to impart to the film infrared absorbability. As infrared absorbers, for example, silica; silicate; hydroxide, oxide, aluminate, borate or sulfate of lithium, calcium, magnesium or aluminium; or hydrotalcite compounds are used.
Of those, hydrotalcite compounds excel in infrared absorbing ability and light transmission when blended in resin, over those of silica; silicate; or hydroxide, oxide, aluminate, borate or sulfate of lithium, calcium, magnesium or aluminium and, therefore, are particularly useful as infrared absorbing agents, and many patent applications have been filed on inventions relating thereto. (Hydrotalcites are complex hydroxide having lamellar structures formed of complex hydroxide layers (base layers) of Mg and Al, separated by interlayer wherein holding anions (e.g., carbonate ions) and water. Those which are represented by the formulae (1) or (4) in the present specification are complex hydroxides having base layers formed of Mg and Al; or Mg, other divalent metal(s) and Al, holding anions and water in the interlayer. Whereas, those represented by the formulae (2) or (5) also are complex hydroxides differing in composition of the base layers, having base layers formed of Li and Al; or Li, other divalent metal(s) and Al, holding anions and water in the interlayer. All of those have structures similar or analogous to those of hydrotalcite, and hence they are collectively referred to as xe2x80x9chydrotalcite compoundsxe2x80x9d in the present specification. Those which are expressed by the formula (1) or (4) are referred to as Mgxe2x80x94Al hydrotalcite compounds, and those of formula (2) or (5), as Lixe2x80x94Al hydrotalcite compounds).
Among patent applications filed in the past on inventions relating to Mgxe2x80x94Al hydrotalcite compounds, there are, for examples, Sho 62 (1987)-31744B-JP (corres. to U.S. Pat. No. 4,686,791 and EP 142,773), Sho 62-53543B-JP, Sho 62-41247B-JP, Sho 63 (1988)-175072B-JP, Sho 63-115743B-JP, Sho 63-149147B-JP, Sho 63-149148B-JP, Sho 64 (1989)-6041B-JP, Hei 4 (1992)-11107B-JP, Hei 6 (1994)-6363B-JP, Hei 6-6364B-JP and Hei 9 (1997)-176390A-JP. Examples of those relating to Lixe2x80x94Al hydrotalcite compounds include: Hei 7 (1995)-300313A-JP (corres. to EP 672,619), Hei 9 (1997)-142835A-JP (corres. to EP 790,214), Hei 9-279124A-JP, Hei 9-800828A (second)-JP (corres. to U.S. Pat. No. 5,767,179 and EP 778,241) [This is the domestic republication of PCT international publication for the patent application. Similar case shall be hereafter marked as xe2x80x9cA (second)xe2x80x9d], Hei 9 (1997)-235420A-JP (corres. to EP 781,800), Hei 10 (1998)-52895A-JP, Hei 10-235776A-JP and Hei 10-226739A-JP.
While the hydrotalcite compounds are expressed by various structural formula in these patent applications, they can be generally represented by the following formula (4) or (5).
(General formula of Mgxe2x80x94Al hydrotalcite compounds):                                                         [                                                                    {                                                                  Mg                        y1                                            ⁢                                              M                        y2                                                  2                          +                                                                                      }                                                        1                    -                    X                                                  ⁢                                                                            Al                      X                                        ⁡                                          (                      OH                      )                                                        2                                            ]                                      (                              base                ⁢                                  xe2x80x83                                ⁢                layer                            )                                            X            +                          ⁢                                            [                                                                    A                                          X                      /                      n                                                              n                      -                                                        ·                  b                                ⁢                                  xe2x80x83                                ⁢                                  H                  2                                ⁢                O                            ]                                      (              interlayer              )                                            X            -                                              (        4        )            
in the above formula,
M2+ stands for at least one kind of divalent metal ion of Zn, Ca and Ni,
Anxe2x88x92 stand for a n-valent anion of, e.g., inorganic or organic acid such Clxe2x88x92, Brxe2x88x92, Ixe2x88x92, NO3xe2x88x92, CIO4xe2x88x92, H2PO4xe2x88x92, HBO32xe2x88x92, SO42xe2x88x92, CO32xe2x88x92, SiO32xe2x88x92, HPO42xe2x88x92, PO43xe2x88x92, Fe(CN)63xe2x88x92 and Fe(CN)44xe2x88x92,
and x, y1, y2 and b are positive numbers each satisfying the following conditions, respectively,
0 less than xxe2x89xa60.5, y1+y2=1, y1xe2x89xa61, y2 less than 1, 0xe2x89xa6b less than 2.
(General formula of Lixe2x80x94Al hydrotalcite compounds)                                                         [                                                (                                                            Li                                              1                        -                        X                                                              ⁢                                          G                      X                                              2                        +                                                                              )                                ⁢                                                                            Al                      2                                        ⁡                                          (                      OH                      )                                                        6                                            ]                                                      (                                  1                  +                  X                                )                            +                                            (                          base              ⁢                              xe2x80x83                            ⁢              layer                        )                          ⁢                                            [                                                                                          (                                              A                                                  n                          -                                                                    )                                                                                      (                                                  1                          +                          X                                                )                                            /                      n                                                        ·                  b                                ⁢                                  xe2x80x83                                ⁢                                  H                  2                                ⁢                O                            ]                                      (              interlayer              )                                                          (                              1                +                X                            )                        -                                              (        5        )            
in which
G2+ stands for at least one kind of divalent metal ion of Mg, Zn, Ca and Ni,
Anxe2x88x92 stands for a n-valent anion,
and x and b are positive numbers each satisfying the following conditions, respectively,
0xe2x89xa6x less than 1, 0xe2x89xa6b less than 5.
Of these, in most cases hydrotalcite compounds having carbonate ions in the interlayer (which are hereafter referred to as carbonate ion-type hydrotalcite compounds) are used.
Taking examples of carbonate ion-type Mgxe2x80x94Al hydrotalcite compounds, however, while they exhibit favorable absorption of infrared rays around 400-800 cmxe2x88x921 and 1400 cmxe2x88x921, the absorbing ability of the infrared rays of 900 to around 1300 cmxe2x88x921 is poor. When they are contained in agricultural film whose base material is polyethylene exhibiting infrared absorption at around 700 and 1300-1500 cmxe2x88x921 only, the agricultural film exhibits combined infrared absorption of that of the polyethylene and that of the infrared absorbing agent and hence shows poor infrared absorption in the vicinity of 900-1300 cmxe2x88x921, i.e., poor heat-insulation property. Also carbonate ion-type Lixe2x80x94Al hydrotalcite compounds show infrared absorbing ability at around 1000 cmxe2x88x921 which however is not strong, and their over-all infrared absorbing ability is about the same as that of carbonate ion type Mgxe2x80x94Al hydrotalcite compounds. Agricultural films containing those compounds, furthermore, are considered to exhibit better light transmission compared to that of the films containing other infrared absorbing agents, but still the light transmission is not fully satisfactory.
As a means to enhance the infrared absorbing ability of Mgxe2x80x94Al hydrotalcite compounds, Sho 62 (1987)-31744B-JP (corres. to U.S. Pat. No. 4,686,791 and EP 142,773) proposed to impart thereto the ability to absorb infrared rays around 900-1300 cmxe2x88x921 by having them contain H2PO4xe2x88x92, HPO42xe2x88x92, PO43xe2x88x92, HBO32xe2x88x92 or SiO32xe2x88x92 and the like, i.e., those normally referred to as silicon-, phosphorus- and boron-containing monomeric oxygen acid ions, in the interlayer. A similar proposal was made also as to Lixe2x80x94Al hydrotalcite compounds. However, when hydrotalcite compounds containing these anions are used as infrared absorbing agents in agricultural film, some improvement in heat insulation of the film is achieved but it is still not fully satisfactory. Furthermore, the ability to impart light transmission to film is either equivalent or inferior to that of conventional carbonate ion-type hydrotalcite compounds.
Recently, proposals for further improving infrared absorption of hydrotalcite compounds are made in Hei 8 (1996)-217912A-JP (corres. to EP 708,056) or Hei 9 (1997)-800828A (second)-JP (corres. to U.S. Pat. No. 5,767,179 and EP 778,241), according to which condensed silicate ions and/or condensed phosphate ions (which are hereafter referred to as silicon- and/or phosphorus-containing polymerized oxygen acid ion) are caused to be present as the interlayer anions of hydrotalcite compounds. The object of these proposals is to improve the infrared absorption by having the compounds contain more silicon- or phosphorus-containing oxygen acid ions in their interlayer, by the use of silicon- or phosphorus-containing polymerized oxygen acid ions. It is furthermore alleged that those methods can approximate refractive indices of the resulting hydrotalcite compounds to those of thermoplastic resins constituting agricultural films and hence can improve light transmission of the films which contain said hydrotalcite compounds. More specifically, while spacing of carbonate ion-type hydrotalcite compounds is about 7.6 xc3x85 at (003) plane or (002) plane, and their refractive index ranges 1.51-1.53, those hydrotalcite compounds shown in Hei 8-217912A-JP (corres. to EP 708,056) or Hei 9-800828A (second)-JP (corres. to U.S. Pat. No. 5,767,179 and EP 778,241), e.g., those having silicon-containing polymerized oxygen acid ions, have increased spacing of 11.9 xc3x85 at the maximum at (003) or (002) planes, and whereby their refractive index decreases to 1.49-1.52. Refractive index of thermoplastic resin useful for agricultural film, e.g., an ethylene-vinyl acetate copolymer, is said to be 1.49-1.50. Hence agricultural films containing the hydrotalcite compounds as exemplified in above two published patent applications are said to exhibit improved light transmission.
However, our reproduction testing of such agricultural films has revealed: although the agricultural films exhibited improved infrared absorption, their light transmission again was at equivalent or inferior level as compared to that of films containing conventional hydrotalcite compounds. While the reason for the absence of improvement in light transmission of the film is not yet fully clear, one of suspected causes is the processing temperature used in the occasion of kneading such a silicon- or phosphorus-containing polymerized oxygen acid ion-carrying hydrotalcite compound into the resin serving as the material for agricultural film, as disclosed in said two published applications.
Thus, none of known infrared absorbing agent in the past could fully satisfy both of the property requirements to have excellent infrared absorption and, when contained in agricultural film, to impart good light transmission to the film.
The object of the present invention is to provide a substance which has excellent infrared absorbing ability and also is capable of imparting excellent light transmission to an agricultural film containing said substance; a process for production thereof, an infrared absorbing agent containing the substance as the active ingredient; and an agricultural film containing said infrared absorbing agent, which concurrently exhibits excellent heat insulation property and excellent light transmission.
We have engaged in research work aiming at accomplishing the above object, to discover that a hydrotalcite compound which is expressed by the following formula (1) or (2) and which holds in its interlayer at least a kind of anions selected from silicon-, phosphorus and boron-containing oxygen acid ions, at least a part of said anions being at least one of silicon-, phosphorus- and boron-containing polymerized oxygen acid ions; and other kind or kinds or anions, exhibits excellent infrared absorbing ability and is capable of imparting excellent light transmission to an agricultural film containing the same. The invention is thus completed. When said hydrotalcite compound is contained in agricultural film as an infrared absorbing agent, a film excelling in heat insulation as well as in light transmission is obtained:
(Mgxe2x80x94Al hydrotalcite compound) 
in which
M2+ stands for at least a kind of divalent metal ion of Zn, Ca and Ni,
A stands for at least a kind of anion selected from silicon-, phosphorus- and boron-containing oxygen acid ions, at least a part of which being at least a kind of anion selected from silicon-, phosphorus- and boron-containing polymerized oxygen acid ions,
B stands for at least a kind of anion other than the A, and
x, y1, y2, z1, z2 and b each satisfies the following condition or conditions:
x: 0 less than xxe2x89xa60.5,
y1 and y2: y1+y2=1, 0 less than y1xe2x89xa61, 0xe2x89xa6y2 less than 1,
z1 and z2: 0 less than z1, 0 less than z2, 
b: 0xe2x89xa6b less than 2.
(Lixe2x80x94Al hydrotalcite compound) 
in which
G2+ stands for at least a kind of divalent metal ion of Mg, Zn, Ca and Ni,
A stands for at least a kind of anion selected from silicon-, phosphorus- and boron-containing oxygen acid ions, at least a part of which being at least a kind of anion selected from silicon-, phosphorus- and boron-containing polymerized oxygen acid ions,
B stands for at least an anion other than the A, and y1, y2, x, z1, z2 and b each satisfies the following condition or conditions:
y1 and y2: 0 less than y1xe2x89xa61, 0 less than y2 less than 1,
0.5xe2x89xa6(y1+y2)xe2x89xa61,
x: x=y1+2y2 
z1 and z2: 0 less than z1, 0 less than z2,
b: 0xe2x89xa6b less than 5.
That is, the hydrotalcite compound of the invention contains between its base layers: as A, at least a kind of anion selected from silicon-, phosphorus- and boron-containing oxygen acid ions, at least a part of which being at least a kind of anion selected from silicon-, phosphorus- and boron-containing polymerized oxygen acid ions (which anions being hereafter referred to as xe2x80x9cA anionsxe2x80x9d) and as B, anion or anions other than A (xe2x80x9cB anionsxe2x80x9d), which characteristically exhibits concurrently excellent infrared absorption and an ability to impart excellent light transmission to agricultural film containing same. Such combination of properties has never been obtained when any known hydrotalcite compounds containing various ions, or those containing silicon- or phosphorus-containing polymerized oxygen acid ions as disclosed in Hei 8-217912A-JP (corres. to EP 708,056) or Hei 9-800828A (second)-JP (corres. to U.S. Pat. No. 5,767,179 and EP 778,241), or their combinations are used as infrared absorbing agent.
The reason for these advantageous properties is not yet 10 fully clear. Whereas, those hydrotalcite compounds carrying silicon- or phosphorus-containing polymerized oxygen acid ions as disclosed in Hei 8-217912A-JP (corres. to EP 708,056) or Hei 9-800828A (second)-JP (corres. to U.S. Pat. No. 5,767,179 and EP 778,241) and which contain a large amount of interlayer water have notably widened spacing as aforesaid, and their refractive index is approximate to that of thermoplastic resins which are used for agricultural films. Due to so widened spacing, according to DTA (differential thermal analysis) the interlayer water is released at temperatures not higher than 150xc2x0 C. On the other hand, in the occasion of blending an infrared absorbing agent into thermoplastic resin to be used for agricultural film, normally they are kneaded at processing temperatures ranging 140-200xc2x0 C. Hence, when said hydrotalcite compound is blended as an infrared absorbing agent in a thermoplastic resin to be used to make agricultural film, the interlayer water in the infrared absorbing agent is released under the processing temperature of 140-200xc2x0 C. to once again narrow the widened spacing. In consequence, its refractive index also largely changes, and eventually when the composition is processed to a film, the film presumably comes to exhibit poor light transmission. This assumption is supported also by the phenomenon that the percent transmission of the film further drops when interlayer water of the infrared absorbing agent is removed in advance of its kneading into the thermoplastic resin and the kneaded composition is processed into film. Furthermore, the infrared absorbing agent contains large amounts of silicon- or phosphorus-containing polymerized oxygen acid ions, which allows a prediction that locally silicate or phosphate compounds are formed inside the crystals (interlayer) during synthesis of the compound or during the release of interlayer water under the heat treatment, and so formed silicate or phosphate compounds may adversely affect light transmission of the product film.
Separately from above assumptions, it is known that interlayer water of hydrotalcite compounds containing anions other than A anions, for example, sulphate ion, carbonate ion, chloride ion or nitrate ion, is released at around 200-240xc2x0 C. The hydrotalcite compound of the invention contains both A anions and B anions and, for example, when it contains as B anions sulphate ion, carbonate ion, chloride ion, nitrate ion and the like, the property of hydrotalcite compound having such ions at its interlayer is added to the hydrotalcite compound of the invention. Consequently, even under the processing temperature of 140-200xc2x0 C. it retains a part of interlayer water to alleviate the narrowing ratio of the spacing and in consequence reduces the change in refractive index. Hence when the compound of the present invention is blended in agricultural film as an infrared absorbing agent, it presumably exhibits little adverse effect on light transmission of the film. Also because the hydrotalcite compound of the invention uniformly contains the plural kinds of anions in the interlayer, presumably formation of silicate compound or phosphate compound scarcely takes place.
The interlayer B anions in the hydrotalcite compound of the invention is at least a kind of anion other than A anions, i.e., other than silicon-, phosphorus- and boron-containing oxygen acid ions, preferably those selected from sulphate ion, carbonate ion, chloride ion and nitrate ion, inter alia, sulphate ion and carbonate ion.
When the compound of the invention is to be contained, for example, in resin, the compound preferably has an average secondary particle diameter of not more than 5 xcexcm and a BET specific surface area of not more than 30 m2/g, for favorable dispersibility. In order to further improve the dispersibility, it may be surface-treated with at least one member of the group consisting of higher fatty acids; anionic surfactants; phosphoric acid esters; nonionic surfactants, silane-, titanate- and aluminum-containing coupling agents; and fatty acid esters of polyhydric alcohols. Also for avoiding occurrence of foaming or fish-eye, the hydrotalcite compound of the invention which has been optionally surface-treated may be partially or entirely removed of the interlayer water by a heat-treatment.
The hydrotalcite compound of the present invention has excellent infrared absorbing ability and the property of imparting excellent light transmission to agricultural film which contains the same, and therefore is suitable as infrared absorbing agent for agricultural films. In particular, referring to the formulae (1) and (2), 20 those compounds whose electric charges fall within the range of 0.1xe2x89xa6(total electric charge number of (B)z1)/xxe2x89xa60.8 are preferred as infrared absorbing agent. Thus, an agricultural film which contains 1-30% by weight of a hydrotalcite compound of the invention to the thermoplastic resin constituting the film concurrently possesses excellent infrared absorbing ability and excellent light transmission.
The hydrotalcite compound of the present invention can be prepared by a process comprising preparing in advance a hydrotalcite compound whose interlayer anions are of at least one kind of anions other than A anions, for example, sulphate ions, carbonate ions, chloride ions, nitrate ions or an organic acid ions, and then exchanging some of them with A anions. In particular, it is preferred to first prepare a hydrotalcite compound whose interlayer anions are of at least one kind of selected from sulphate ion, carbonate ion, chloride ion and nitrate ion, and then exchanging a part of them with A anions. The optimum result can be obtained, furthermore, by preparing a hydrotalcite compound containing mainly sulphate ions at the time of the synthesizing reaction and then exchanging some of the sulphate ions with A anions, because it can be prepared with ease and at low cost, and furthermore it assists the infrared absorbing ability which shall be discussed later.